feat: add an example of how to package & integrate custom plugins

[effervescentia]

Brief description. What is this change?

Add an example detailing how to bundle a JavaScript snippet that adds custom functionality to Voiceflow's webchat.

Checklist

• [ ] Breaking changes have been communicated, including:
  • New required environment variables
  • Renaming of interfaces (API routes, request/response interface, etc)
• [ ] New environment variables have been deployed
• [ ] Appropriate tests have been written
  • Bug fixes are accompanied by an updated or new test
  • New features are accompanied by a new test